The present invention relates generally to read/write transducer heads, and in particular to measuring thin-film write head DC head voltage in a disk drive.
In recording of data on magnetic media with transducer heads, such as in disk drive data storage systems, the magnetic recording performance is sensitive to write current amplitude in the head. As data rates and recording densities are pushed to the limits of technology it becomes crucial to overall cost and performance to extend the performance of the overall recording system.
As shown by example in FIG. 1A, in disk drives with magneto-resistive (MR) heads each transducer head 10 includes a thin-film writer element (write coil) 12 connected to a writer driver 14 in a preamplifier (Preamp) 16 via an interconnect 18, and a read element (MR Read Sensor) 20 connected to a read amplifier 22 in the preamplifier 16, via the interconnect (transmission line) 18.
The write driver 14 employs a low-impedance output stage followed by a series termination resistance to drive the write current IWC out to the write element 12. The series termination resistance is used to terminate the transmission line 18 from the write driver 14 out to the thin-film write element 12. The write current amplitude IWC in the transducer head generated by the write driver 14 is one of the critical parameters for magnetic recording. A consequence of using the low-impedance output buffer in the write driver 14 is that the amplitude (magnitude) of the write current is sensitive to the DC resistance of the thin-film write element 12 as well as the interconnects 18, and the absolute value of the termination resistance. In order to more accurately predict the magnitude of the write field output from the head 10, it is necessary to optimize (calibrate) the output voltage of the low-impedance write driver 14, and hence the write current IWC.
Further, if the amplitude of the write current is not well controlled across a population of transducer heads 10, preamplifiers 16 and interconnects 18, then the overall system performance versus cost is subject to the limits imposed by the worst case (lowest) write current. The lowest write current of a population of transducer heads 10 results from the combination of the highest resistance of each of the following components: the thin-film writer 12, the interconnect 18 and the termination resistance of the voltage-mode (matched-impedance) write driver 14. In conventional disk drives, over-driving the remaining population of heads 10 can result is a number of problems with the write process such as poorly defined write field as a result of over-driven heads, excess power consumption within the preamplifier 16, and increased write-to-read coupling.
There is a need, therefore, for a method and apparatus for calibrating the write current in a transducer head. There is also a need for a method of calibrating output voltage of low-impedance write drivers to generate the desired write current. There is also a need for a method of overcoming the uncertainty in write current amplitude that results from the use of conventional voltage-mode or matched-impedance write driver architecture.
The present invention satisfies these needs by providing method and apparatus to optimize (calibrate) the low-impedance write driver voltage, and hence the write current. The present invention allows measurement of a thin-film write head DC head voltage within a disk-drive, and calibration of the write current based on the measured voltage.
In one embodiment, the present invention provides a technique and associated circuitry for measuring the buffered write head voltage (e.g., measurement of thin-film write head DC head voltage) along with a procedure for controlling (calibrating) the write current amplitude generated by the write driver based on the measurement of the buffered write head voltage. The write driver includes a terminating resistor, coupled to the write element of a transducer head in a disk drive, wherein controlling the write current of the write driver includes the steps of: measuring the resistance RW of the terminating resistor; measuring the resistance RC of the write element; and generating the write current amplitude based on the resistances RC and RW.
In one example, the write driver further includes a voltage source and a switch for electrically connecting the voltage source in series with the terminating resistor RW and the write element, such that the steps of measuring RW and RC further include the steps of: with the voltage source disconnected, connecting a current source (IWB) across the parallel combination of the terminating resistor and the write element, and measuring a resulting first voltage across the parallel combination of the resistances RC and RW; with the current source disconnected, connecting the voltage source (VW) in series with the terminating resistor and the write element, and measuring a second voltage across the series combination of the resistances RC and RW; and determining the values of RC and RW based on said two measured first and second voltages. Generating the write current further includes the steps of adjusting the voltage VW of the voltage source to generate a desired write current IWC based on the resistances RC and RW, such that: IWC=VW/(RC+RW).
Conventionally, the write current amplitude is subjected to an unknown variation due to process tolerances in the head and preamplifier. These variances can degrade the write process by an unknown disturbance function. The present invention overcomes the uncertainty in setting the write current amplitude that results from the use of a voltage-mode or matched-impedance write driver architecture, by providing a method and practical circuits for calibrating the write current amplitude against a current source (IWB), wherein the current source (IWB) is based on a precise reference current (REF) independent of the DC resistance of the thin-film write head. As such, according to the present invention, the write current amplitude can be accurately controlled without sensitivity to the DC resistance of the head plus interconnect resistance (RC) or the output impedance of the write driver (RW).